1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) devices.
2. Description of the Background Art
SAW devices include SAW oscillators, transversal SAW filters and oscillator type SAW filters, which are used for different purposes. In the following, a SAW filter will be described as one type of a SAW device.
FIG. 17 is a perspective view schematically showing a structure of a SAW filter. Referring to FIG. 17, SAW filter 15 basically includes a piezoelectric substrate 12, and a four-terminal structure formed on a surface of piezoelectric substrate 12 and having two pairs of comb-shaped electrodes 13 respectively used for exciting and receiving surface waves. Such electrodes 13 are called interdigital electrodes, and this type of transducer is called an IDT (Interdigital Transducer).
Referring to FIG. 18, generally, when an impulse voltage is applied to comb-shaped electrodes 13 for excitation, strains of opposite phases are caused between adjacent electrodes 13 by a piezoelectric effect, thereby exciting a SAW. The SAW propagates on the surface of piezoelectric substrate 12. The strains caused by the SAW produce electric charges at the surface of piezoelectric substrate 12, which are, in turn, received as electric signals by comb-shaped electrodes 13 for reception.
Conventionally, the SAW device such as SAW filter 15 has a structure in which electrodes 13 in accordance with the purpose of the device are formed on the surface of piezoelectric substrate 12, as shown in FIG. 19. The characteristic of SAW device 15 depends largely on the characteristic of piezoelectric substrate 12, which is also used according to the purpose of the device. Table 1 shows typical materials used for piezoelectric substrate 12 and characteristics of an SAW propagating on piezoelectric substrate 12.
TABLE 1 Characteristics of Substrate for Typical SAW device propagation propa- Eulerian angles velocity K.sup.2 TCD gation substrate .phi., .theta., .PSI. [m/s] [%] [ppm/.degree. C.] mode quartz 0.degree., 132.75.degree., 3159 0.12 0 Rayleigh 0.degree. wave 0.degree., 15.degree., 0.degree. 3948 0.11 0 Leaky wave LiTaO.sub.3 90.degree., 90.degree., 112.degree. 3328 1.1 16.5 Rayleigh wave 0.degree., 126.degree., 0.degree. 4211 4.7 45.1 Leaky wave LiNbO.sub.3 0.degree., 38.degree., 0.degree. 4007 5.2 71.4 Rayleigh wave 0.degree., 154.degree., 0.degree. 4731 10.9 61.3 Leaky wave
As shown in Table 1, a quartz substrate has a good temperature characteristic (value near zero), but the electromechanical coupling factor (K.sup.2) is disadvantageously small. For an LiNbO.sub.3 (LN) substrate, although K.sup.2 is sufficiently high, the temperature characteristic such as a temperature coefficient of a delay time (TCD), is disadvantageously high. Substrates using LiTaO.sub.3 (LT) conventionally include an X-112.degree. Y LT substrate (LT of 90.degree., 90.degree., 112.degree.) in an Eulerian angles representation). The conventional LT substrate has a characteristic intermediate between those of a quartz substrate and the LN substrate.
Thus, the substrates have their own advantages and deficiencies, so that they are used according to the specific purposes of the device. Recently, with technological developments in the field of display units including televisions and telecommunication apparatuses including portable telephones, SAW devices used therefor are also required to have enhanced properties.
Now, Eulerian angles in the above Table 1 will be described with reference to FIG. 20.
Referring to FIG. 20, the X axis is rotated by an angle .phi. toward the Y axis about the Z axis, and the axis obtained is defined as the first axis.
Then, the Z axis is rotated counterclockwise by an angle .theta. about the first axis, and the axis obtained is defined as the second axis. A material is cut in accordance with a surface orientation along a plane including the first axis and having the second axis as the normal, and used as a substrate. On the substrate which has been cut in accordance with the above mentioned surface orientation, the first axis is rotated counterclockwise by .psi. about the second axis, and newly defined as the third axis. The third axis corresponds to a direction in which the SAW propagates. An axis orthogonal to the third axis on the plane is defined as the fourth axis. Thus, the Eulerian angles (.phi., .theta., .psi.) are defined.
A center frequency f.sub.0 of a SAW device is determined in accordance with the following equation: EQU f.sub.0 =V/.lambda.
(V: propagation velocity of the SAW, .lambda.: electrode pitch of the IDT (FIG. 17))
Thus, in producing devices having the same center frequency f.sub.0, if the LT substrate with relatively high propagation velocity V as compared with that of the quartz substrate or the like, the electrode pitch of the IDT .lambda. increases and the size of the SAW device is increased.
It is commonly believed that the higher K.sup.2 a piezoelectric substrate has, the easier is the design of a device with a large bandwidth. However, the LN substrate with a high K.sup.2 is accompanied by a high TCD, and is not suitable for a device which should have a good temperature characteristic.